With dramatic and devastating hurricanes plastered across the Atlantic Ocean as much as they are sprinkled across your news feeds and your television screens, a certain question may have come up: Just how powerful can these hurricanes get?
This is a question that no one has a concrete answer to, but that doesn’t mean that academics – and us at IFLScience – haven’t pondered on it. In short, it appears that there is no upper limit to how powerful these tropical beasties can get, but that really depends on what you mean by "powerful".
Wind Power Run Amok
Hurricanes are usually measured, in terms of power, in two ways: wind and pressure. The higher the wind speed and the lower the central pressure, the more energetic the hurricanes (or typhoons) will be. Generally speaking, though, wind strength is the yardstick by which all tropical storms are measured.
Enter, the Saffir-Simpson Hurricane Wind Scale (SSHWS), which was first developed in 1971 by a civil engineer and a meteorologist at the National Hurricane Center. For several decades, the peak height of the storm surge – the “tsunamis” that are brought ashore by hurricanes – and the central peak pressure were used along with wind strength to categorize hurricanes, but as of 2010, wind alone defines them.
The scale is relatively simple:
Tropical Depression = less than 63 km/h (less than 39 mph)
Tropical Storm = 63-118 km/h (39-73 mph)
Category One Hurricane = 119-153 km/h (74-95 mph)
Category Two Hurricane = 154-177 km/h (96-110 mph)
Category Three Hurricane = 178-208 km/h (111-129 mph)
Category Four Hurricane = 209-250 km/h (130-156 mph)
Category Five Hurricane = greater than 252 km/h (>156 mph)
Irma, for example, was a Category 5 hurricane. With peak sustained wind strengths of 298 km/h (185 mph), it is the most powerful Atlantic Ocean hurricane in recorded history. It is, however, not the most powerful of all time. That accolade, when it comes to wind speed alone (and not central pressure), goes to 2015’s Hurricane Patricia, which peaked with winds at 345 km/h (215 mph) in the Eastern Pacific Ocean.
(As a brief note, this scale is only officially used to measure in the Northeast Pacific and the Atlantic. In other parts of the world, they are known as typhoons, and one with wind speeds of 241 km/h [150 mph] is known as a super typhoon. Still, the same wind speed principles apply.)
So why no Category 6? Why is Category 5 left as the upper, unbounded section?
According to Robert Simpson, one of the brainchildren behind the SSHWS, the reason that Category 5 has no successor is that the scale is meant to infer the damage a hurricane would cause to human infrastructure. Almost any building experiencing sustained Category 5 winds simply won’t survive.
Still, this leaves open the possibility that you could get stronger hurricanes. So what does the science behind hurricanes have to say about all this?
The wind speed of a hurricane is determined by the pressure of the hurricane – here’s how.
Warmer ocean water has a higher evaporation rate, which produces water vapor. As the water vapor condenses into rain and clouds, it releases heat, which warms the surrounding air. Warmer air expands, which lowers the pressure. The warmer the air mass is, the lower the central pressure will be.
This local low-pressure zone causes the high-pressure surroundings to attempt to compensate for this; winds begin to rush into the center of the depression, and as the Earth is rotating, these winds begin to spiral. Eventually, if there’s enough latent heat released, you get a hurricane.
Based on this mechanism then, it’s reasonable to assume that the warmer the surface water is, the stronger the peak winds will be.
Although there are plenty of complicating factors, and the datasets before 1970 are somewhat unreliable, it appears that the scientific basis for windier hurricanes is there. There is a link between sea surface temperatures and wind speeds, but the exact numbers have yet to be nailed down.
Theoretically though, as long as the oceans warm, then there is once again no upper limit on the peak wind strength of hurricanes. Hurricanes have already pointed this out to us: Although not sustained wind, a single gust generated by 1996’s Tropical Cyclone Olivia clocked in at 407 km/h (253 mph), almost the same as those at the fringes of Jupiter’s Great Red Spot.
The C Words
Wind speed, of course, isn’t the only way to measure the power of a hurricane. What if we threw out the SSHWS and took the rain and floods into account?
After all, for the average hurricane, roughly 400 times more energy goes into cloud and rain formation than it does in generating those terrifying winds. Judging by the mind-blowing amount of water generated by Harvey, it seems as if the focus on wind speed alone is, if you’ll forgive me, a little overblown.
The amount of water that air is able to hold is based on a marvelous bit of mathematics known as the Clausius-Calpeyron equation, which states that there is a 3 percent increase in average atmospheric moisture content for each 0.5°C (0.9°F).
Assuming the atmospheric pressure doesn’t drastically change, then once again, there’s no real upper limit to this equation. The more we warm the planet, the worse hurricanes will be.
If there is a business-as-usual global warming scenario – a 4°C (7.2°F) rise by 2100 – then there will be roughly 24 percent more water in the hearts of hurricanes than there should have been. The hurricanes generated then will make Harvey’s legacy look like a puddle.
So, if by “more powerful” you mean “wetter”, then it looks like the future will be full of life-threatening and record-breaking floods.
Hurricanes also produce phenomena known as storm surges. As these storms are low-pressure monsters, they are able to suck up the sea with them as they near land. Their powerful winds, however, do most of the work in causing seawater to rush onto the shore. Global sea levels are rising at a remarkable rate, which means that each year, the storm surges of hurricanes are higher than they should be.
As there’s also no real limit to sea level rise, again, in this sense, hurricanes will continue to get more powerful as the climate continues to change.
The Shrinking Horizon
Hurricane intensity, thanks to climate change, will increase by 2 to 11 percent by 2100. That may not sound like too much, but that doesn’t take into account the Power Dissipation Index (PDI), a new measure that considers the frequency, intensity, and duration of hurricanes over time.
The PDI looks at the amount of energy unleashed during a hurricane, rather than just its wind speed, and some would argue that this makes a better proxy for destructive potential. According to NOAA, the PDI of Atlantic hurricanes will increase by a whopping 300 percent by 2100, which means the future will be full of wind and flood-swept ruins.
So how powerful can hurricanes get? It looks like we’re not that far off from finding out.